Trophic ecology of the neustonic cnidarian Velella velella in the northern California Current during an extensive bloom year: insights from gut contents and stable isotope analysis

Aggregations of the neustonic hydrozoan Velella velella occur periodically in the northern California Current. Despite the regular occurrence of notable bloom events in this productive upwelling zone, little is known about their trophic ecology. We used gut content and stable isotope analyses (SIA) to elucidate V. velella prey selectivity and trophic niche to address their potential impacts on the marine ecosystem. The dominant prey items ingested by V. velella colonies were non-motile prey including cladocerans and northern anchovy (Engraulis mordax) eggs, though copepods were also common in gut contents. Removal rates of northern anchovy eggs could be magnified in bloom years and in areas of high spawning biomass. Stable isotope analysis revealed differences in isotopic niche width and overlap among V. velella based on latitudinal gradients and to a lesser extent on V. velella size and demonstrates the need for continued work to fully understand the trophic ecology of this unique neustonic organism.